1. Field of the Invention
This invention relates to a current mirror circuit suitable for use with a lower voltage power supply.
2. Description of Related Art
Current mirror circuits have previously comprised MOS (Metal Oxide semiconductor) transistor and used with various semiconductor circuits. FIG. 1 illustrates static characteristics of an NMOS transistor. The horizontal axis indicates the drain source voltage Vds applied to an NMOS transistor and the vertical axis indicates the drain current Id. The relation between Id and Vds is shown as the gate source voltage Vgs changes. The dotted line in FIG. 1 represents a boundary of two regions that exist between Id and Vds. One region is on the left side of the dotted line is called the triode region, where Id is represented by equation I.
When (Vgsxe2x88x92Vt) greater than Vds,
Id=xcex2[(Vgsxe2x88x92Vt)Vdsxe2x88x92xc2xdVds2]xe2x80x83xe2x80x83(I) 
Where, Vt is threshold voltage of the MOS transistor.
The other region is on the right side of the dotted line and is called the pentode region, where Id is represented by equation II.
When (Vgsxe2x88x92Vt) less than Vds,
Id=xc2xdxcex2(Vgsxe2x88x92Vt)2xe2x80x83xe2x80x83(II) 
The dotted line by which divides these two regions is represented by equation III.
Vgsxe2x88x92Vt=Vdsxe2x80x83xe2x80x83(III) 
Moreover, when the conditions of equation IV occur, the NMOS transistor hardly allows current to flow.
Vgs less than Vtxe2x80x83xe2x80x83(IV) 
A similar relationship also occurs in a PMOS transistor. FIG. 2 shows a circuit where the two NMOS transistors M0 and M1 are connected, where the length of the gate and the width of the channel of both NMOS transistors M0 and M1 are equal.
Because the gate terminal and the drain terminal are short-circuited, the NMOS transistor MO operates within the range of the pentode region regardless of the current flow of constant current source 101. The gate-source voltage of NMOS transistor M1 is equal to the voltage between the gate and the source of M0. Therefore, when the drain-source voltage is sufficiently high, NMOS transistor M1 operates within the range of the pentode region. This circuit is called a current mirror circuit because it is used to make the drain current of NMOS transistor M1 equal to the drain current of NMOS transistor M0.
In this current mirror circuit of related art, the current flowing in NMOS transistor M1 decreases when drain-source voltage of the transistor M1 decreases, and the transistor M1 begins to operate in triode region. As a result, the current value that flows in NMOS transistor M0 differs from that of NMOS transistor M1, and the current mirroring deteriorates.
Recently, semiconductor circuits have been required to operate on lower supply voltages. When current mirror circuits such as the one shown in FIG. 2 operate on a lower supply voltage, the drain-source voltage of the NMOS transistor M1 drops and the operation margin of the current mirror decrease.
In the pentode region,
Vgsxe2x88x92Vt less than Vdsxe2x80x83xe2x80x83(V) 
Then, it is possible to avoid this problem by lowering the threshold voltage of Vt for MO and M1. However, the circuits having transistors which have a lowered threshold voltage are excessively costly to manufacture.
Moreover, the drain current of the pentode region is shown more accurately by the next expression.
When (Vgsxe2x88x92Vt less than Vds),
Id=xc2xdxcex2(Vgsxe2x88x92Vt)2(1+xcexVds)xe2x80x83xe2x80x83(VI) 
where xcex is a fitting parameter.
Even if NMOS transistor M1 operates in the pentode region, an accurate current mirroring cannot be obtained because the drain current of M1 has dependency on the drain-source voltage. To address this problem the circuit shown in FIG. 3 has been proposed. NMOS transistors are placed in series in order to suppress changes of the drain voltage of transistor M11, which mirrors the current. Decreasing operation margin associated with lower supply voltages has occurred since connecting a compensation means such as transistor M11 to a mirror current in series and this technique runs counter to the trend of using lower voltages for semiconductor circuits.
One object of this present invention is to solve the above-mentioned problems of the prior art by providing a current mirror circuit that can increase the lower supply voltage operation margin of the current mirror operation, thereby obtaining an excellent current mirror circuit, even with a low-voltage power supply, and alleviating the drain-source dependency of the mirror current.
According to one aspect of the present invention, a circuit that provides an excellent mirror current that does not deteriorate, even when the power source becomes lower supply voltage. In a presently preferred embodiment, A mirror current flows in a first MOS transistor when a constant current flows in the MOS transistor from a current source. An operational unit outputs the difference between voltage Vg1 of the gate of the MOS transistor and voltage Vd1 of the drain, and applies this difference to the gate of a second MOS transistor. When the power-supply voltage of this circuit becomes lower and the absolute value of Vd1 decreases, the MOS transistors enter the triode region, and the mirror current decreases. When the absolute value of Vd1 decreases, because the difference between Vg1 and Vd1 becomes larger, the drain current of the second MOS transistor increases, and the amount by which the mirror current decreases is counterbalanced.